The discovery of basophils in delayed hypersensitivity reactions has been quickly followed by findings indicating that basophil-rich reactions are themselves probably heterogeneous. Our findings indicate that basophil infiltrates can be an aspect of a variety of reactions with a delayed time course, and that these basophil accumulations are under regulation. The arrival of these cells in some instances is mediated by antibodies, while in other instances basophil arrival is controlled by T cell mediated factors. There are undoubtedly other factors controlling the number of basophils found at delayed reactions, and our studies also indicate that the function of cells containing vasoactive amines at delayed reactions can be governed by antibodies, and also by T cells. It is thus evident that delayed reactions which were heretofore thought to be solely mediated by sensitized T cells releasing non-specific lymphokines able to influence macrophages, are far more complex mixtures in which T cell-lymphokine-basophil and mast cell interactions are important; and antibody-basophil mechanisms are yet another new component. BIBLIOGRAPHIC REFERENCES: Askenase, P.W., Haynes, J.D., and Hayden, B.J.: Antibody mediated basophil accumulations in cutaneous hypersensitivity reactions of guinea pigs. Accepted by J. Immunol. 116: 1976. Askenase, P.W., and Higashi, G.I.: Cutaneous basophil hypersensitivity and macrophage migration inhibition in guinea pigs with Schistosomiasis. Clin. Exp. Immunol. 23:318, 1976.